Photothermography is an established imaging technology. In photothermography, a photosensitive media is exposed to radiation to create a latent image which can then be thermally processed to develop the latent image. Devices and methods for implementing this thermal development process are generally known and include contacting the imaged photosensitive media with a heated platen, drum or belt, blowing heated air onto the media, immersing the media in a heated inert liquid and exposing the media to radiant energy of a wavelength to which the media is not photosensitive, e.g., infrared. Of these conventional techniques, the use of heated drums is particularly common.
A common photosensitive media usable in these imaging processes is known as a photothermographic media, such as film and paper. One photothermographic media has a binder, silver halide, organic salt of silver (or other reducible, light-insensitive silver source), and a reducing agent for the silver ion. In the trade, these photothermographic media are known as dry silver media, including dry silver film.
In order to precisely heat exposed photothermographic media, including film and paper, it has been found to be desirable to use electrically heated drums. In apparatus employing this technique, a cylindrical drum is heated to a temperature near the desired development temperature of the photothermographic media. The photothermographic media is held in close proximity to the heated drum as the drum is rotated about its longitudinal axis. When the temperature of the surface of the heated drum is known, the portion of the circumference around which the photothermographic media is held in close proximity is known and the rate of rotation of the drum is known, the development time and temperature of the photothermographic media can be determined. Generally, these parameters are optimized for the particular photothermographic media utilized and, possibly, for the application in which the photothermographic media is employed.
In order to achieve a high quality-image in the photothermographic media, very precise development parameters must be maintained. Generally, the circumference of the drum over which the photothermographic media travels will not vary significantly. Also, the rate of rotation of the drum, or the transport rate of the photothermographic media through the thermal processor, can be rather precisely maintained. However, it is generally more difficult to control and maintain the temperature of the surface of the drum.
In addition, other factors also contribute to inaccurate processing. The closeness of the proximity which the photothermographic media is held to the drum partially determines the temperature at which the emulsion in the photothermographic media is heated. Further, the presence of foreign particles between the drum and the photothermographic media can interrupt the flow of heat from the drum to the photothermographic media which can affect image quality.
Because many factors affect image quality, one of which is the temperature at which the photothermographic media is developed, the preciseness at which the surface temperature of the drum can be maintained is important to thermal processing of photothermographic media.
The temperature of the drum depends upon many factors. These include the rate at which heat is delivered to the drum, the thermal conductivity and the thermal mass of the drum, the thermal mass of the photothermographic media, the rate, i.e., the number of sheets (if sheet photothermographic media is used) of photothermographic media being processed, the ambient temperature, whether thermal processing is just beginning or whether the thermal processing is in the middle of a long run.
In addition, heated drums are used extensively in various other material processing applications. Examples include calendaring, laminating coating and drying.
Typically, heat is delivered to such drums through the use of electrical resistance heating elements. Since the heated drum is rotating during thermal processing and since it is desirable to deliver electrical power to the electrical resistance during rotation of the drum, it is desirable to be able to deliver electrical power from a stationary power source, e.g., the standard AC line, to the moving, rotating, drum. Electrical power may be delivered to the drum through the use of slip rings coupled to the drum.
In addition, to precisely control the temperature of the electrically heated drum there should be a means to sense the temperature of the drum and a means to control the electrical power applied to the electrical resistance heaters in response to the signal from the temperature sensor.
While temperature control techniques and apparatus are common, the use of such techniques and apparatus on moveable objects or rotating drums is made more difficult by movement of the object or the rotation of the drum.
One solution has been to locate all temperature sensing and control techniques on the moveable object or rotating drum. In the case of a rotating drum, analog temperature control techniques have been used by incorporating a circuit board containing the analog circuitry on or near the rotating drum allowing the circuit board to rotate along with the drum. While this technique minimizes the difficulty of communicating temperature sensing information and control information between the drum and the analog circuitry, it makes it more difficult to interface to the analog control circuitry or to change or adjust the temperature or control algorithm.
A similar technique, employed by Systek, Minneapolis, Minn., utilizes rotating temperature control circuitry and additionally provides a technique for the communication of sensed temperature information from the rotating drum/control circuitry and the communication of adjustment parameters from the user of the thermal processor utilizing the drum to the rotating drum/control circuitry. A ring of a plurality of light emitting diodes are arranged in a generally circular pattern on one end of the drum/control circuitry. A single light emitting diode is positioned on a stationary board near to that one end of the rotating drum/control circuitry. A light sensor is located on the rotating drum/control circuitry on the one end on the axis of rotation. Similarly, a second light sensor is located on the stationary board. Each light sensor is adapted to sense the duty cycle modulated pulse train of the corresponding light emitting diode(s) on the opposite member. Interference in light transmission is minimized by having each pair of light emitting diodes and sensors act at a different frequency. For example, one pair could operate in the visible spectrum and the other pair could operate in the infrared spectrum.
However, the Systek system is limited to the reading of rather coarse temperature sensing information. Further, the entire temperature control loop circuitry is entirely located on the rotating drum/control circuitry board. Thus, any intelligence built into the temperature control loop must be able to be contained on the rotating drum/control circuitry board, limiting the power and options available.